1. Field of the Invention
The invention generally relates to construction of optics and more particularly to liquid crystal displays.
2. Description of the Related Art
In a liquid crystal display, the display can be controlled electrically to vary between two predetermined levels of transparency. At one level of transparency, the liquid crystal display is nearly transparent, resulting in a light state. At the second level of transparency, the liquid crystal display is nearly opaque, resulting in a dark state.
The liquid crystal in a liquid crystal cell or display is encased in a relatively rigid material which gives it shape. The liquid crystal in a liquid crystal display has an extraordinary axis which has associated with it an extraordinary index of refraction (usually high), and an ordinary axis, which usually has associated with it an ordinary index of refraction lower than the extraordinary index of refraction. When light transmits through the liquid crystal, the rate at which the light passes through the liquid crystal depends on the polarization of the light. Light polarized along the ordinary axis of the liquid crystal passes through at a speed consistent with the ordinary index of refraction. Light polarized along the extraordinary axis of the liquid crystal passes through at a speed consistent with the extraordinary index of refraction. Light polarized such that it travels between the two axes has a component that travels along the ordinary axis and a component that travels along the extraordinary axis.
In the light state, the phase difference between light traveling along the ordinary axis and light traveling along the extraordinary axis results in a final polarization state that is transmitted. In the dark state, the phase difference between light traveling along the ordinary axis and light traveling along the extraordinary axis results in a polarization state that is blocked. The difference between the two states arises from a change in the orientation of the liquid crystal molecules resulting from a voltage applied to the liquid crystal.
The intensity of the light transmitted through a liquid crystal cell, such as an electrically controlled birefringence (ECB) cell, between crossed polarizers, may be expressed roughly as:
I=Imax sin2(2xcfx80xcex94nd/xcex)
xcex represents the wavelength of the light in question, xcex94n is the difference in refractive indices of the LC (also called the birefringence), d is the thickness of the LC layer. It will become apparent that the dark state results when xcex94nd is zero. And is an expression of the phase change induced due to the difference between the two indices of refraction of the liquid crystal, and the distance the light travels in the liquid crystal.
The effective xcex94n of the LC cell can be controlled by an applied voltage signal. By forming electrodes on one or both surfaces of the LC cell, individual pixel regions can be controlled to make a display. It should be understood that an LC display uses a special case of the LC cell, with individual pixels being addressable due to the layout of electrodes.
Unfortunately, as xcex94n approaches 0 in an LC, it changes approximately in proportion to 1/V, where V is the voltage applied to the liquid crystal. Therefore, achieving a xcex94nd value of close to zero may require an excessively high voltage. As a result, a retarder is added to the liquid crystal cell, as illustrated in FIG. 1.
FIG. 1 illustrates a prior art embodiment of a retarder and liquid crystal (as a liquid crystal cell or display) combination. The polarizer 110 causes the light passing through it to be polarized along a first axis which may be referred to as a zero degree axis. The retarder 120 has an extraordinary (slow) axis similar to the liquid crystal, and that extraordinary axis is aligned at a xe2x88x9245 degree angle from the zero degree axis of the polarizer 110. Placed next to the retarder 120 is the liquid crystal 130, which has its extraordinary axis aligned at a +45 degree angle from the zero degree axis of the polarizer 110. The retarder has a fixed value for xcex94nd which may be denoted xcex93. Therefore, the intensity of light passing along the path illustrated by ray 140 after passing through the analyzing polarizer 150 may be calculated as approximately:
I=Imax sin2(2xcfx80(xcex94ndxe2x88x92xcex93)/xcex).
By introducing the retarder, the voltage V applied to the liquid crystal may be adjusted to a more desirable value because the display can now give a nearly perfect dark state when xcex94nd of the display is not zero, thus allowing the 1/V relationship mentioned earlier to be satisfied more easily. Note that the polarizers such as polarizer 110 and analyzing polarizer 150 may be implemented with a variety of materials (thus forming polarizing elements) and may be coupled to the combination optically rather than physically.
However, it will be appreciated that in some circumstances, finding a retarder that fits the exact specifications required for the above relationship may be difficult. Moreover, retarders often have significant variations in the value of xcex94nd within the manufacturer""s specifications for the retarders. Therefore, even when the correct value of xcex94nd is expected, it may not be achieved.
A controlled angle retarder and method for making is presented. In one embodiment, the invention is a liquid crystal display. The liquid crystal display includes a liquid crystal cell having an extraordinary axis. The liquid crystal display also includes a first retarder connected to the liquid crystal cell, the first retarder having an extraordinary axis. The extraordinary axis of the first retarder is aligned at an angle to the extraordinary axis of the liquid crystal cell; the angle is sufficient to produce a desired effective retardance of the first retarder within the display different from a specified retardance of the first retarder.
The invention in an alternate embodiment is a method of making a liquid crystal display (LCD). The method includes measuring a retardance of a retarder. The method also includes calculating an alignment angle of the retarder such that the retarder will have a desired effective retardance, in combination with another optical element in the LCD, which is different from the retardance measured for the retarder. The method further includes optically coupling the retarder to a liquid crystal cell in an alignment using the alignment angle previously calculated, the alignment angle used to adjust an angle between an extraordinary axis of the retarder and an extraordinary axis of the liquid crystal cell.
The invention in another alternate embodiment is a method of making a liquid crystal combination. The method includes connecting a retarder to a liquid crystal cell loosely. The method also includes measuring a combined retardance of the retarder and liquid crystal cell. The method further includes determining whether the combined retardance is suitable and adjusting an alignment angle of the retarder to the liquid crystal cell.